U.S. patent number 3,762,136 [Application Number 05/158,979] was granted by the patent office on 1973-10-02 for preparation of asymmetric polymer membranes.
This patent grant is currently assigned to General Electric Company. Invention is credited to Shiro G. Kimura.
United States Patent |
3,762,136 |
Kimura |
October 2, 1973 |
PREPARATION OF ASYMMETRIC POLYMER MEMBRANES
Abstract
A method is disclosed for the preparation (by the utilization of
a proper solvent system) of dry asymmetric membranes comprising a
porous layer of interconnected crystals of polymer material.
Membranes of many polymer materials may be optionally prepared
either with or without a dense surface layer as one face thereof.
In either case the porous layer is structured with graded porosity.
A three-component casting solution is prepared containing the
polymer, a first good volatile solvent for the polymer and
(relative to the first solvent) a poor less-volatile solvent for
the polymer, which is miscible with the good solvent. A membrane is
cast, allowed to desolvate for a short time and is then immersed in
a leaching agent, that is miscible with both the aforementioned
solvents but is a non-solvent for the polymer. The membrane is then
permitted to dry.
Inventors: |
Kimura; Shiro G. (Schenectady,
NY) |
Assignee: |
General Electric Company
(Schenectady, NY)
|
Family
ID: |
27365117 |
Appl.
No.: |
05/158,979 |
Filed: |
July 1, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
36923 |
May 13, 1970 |
3709774 |
|
|
|
Current U.S.
Class: |
96/13 |
Current CPC
Class: |
B01D
67/0013 (20130101); B01D 69/02 (20130101); B01D
67/0011 (20130101); B01D 71/52 (20130101); C08J
9/28 (20130101); C08J 2201/0546 (20130101); Y10T
428/268 (20150115); C08J 2201/0543 (20130101); Y10T
428/249989 (20150401); B01D 2323/12 (20130101); Y10S
264/14 (20130101); Y10T 428/249979 (20150401); B01D
2325/022 (20130101); Y10T 428/249961 (20150401) |
Current International
Class: |
B01D
71/52 (20060101); B01D 69/00 (20060101); B01D
69/02 (20060101); B01D 67/00 (20060101); B01D
71/00 (20060101); C08J 9/00 (20060101); C08J
9/28 (20060101); B01d 053/22 () |
Field of
Search: |
;55/16,158 ;264/41 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Hart; Charles N.
Parent Case Text
This is a division application of U. S. Pat. application Ser. No.
36,923 -- Kimura (now U.S. Pat. No. 3,709,774), filed May 13, 1970,
and assigned to the assignee of the instant invention.
Claims
I claim
1. In an apparatus for altering the composition of a mixture of
gases, the apparatus comprising in combination a chamber, a
permeable membrane forming at least a portion of the wall area of
the chamber and means in communication with the chamber for
conducting gas thereto for contact with the chamber side of the
permeable membrane, the improvement in said combination comprising
the permeable membrane being a dry polyarylene oxide membrane
having a porosity increasing progressively from adjacent one major
surface of the membrane to the second major surface thereof, the
porosity being due to the presence of pores extending through a
mass of polymer crystals interconnected in a relatively stiff
system, said pores being microporous and interconnected, and the
one major surface adjacent the region of lesser porosity alone
being in the form of a thin, dense, non-porous skin formed integral
with and supported by the porous portion of said membrane, said
skin and said porous portion both being polyarylene oxide
polymer.
2. The improvement as recited in claim 1 wherein the polyarylene
oxide is polyxylylene oxide and the non-porous skin is less than
about 3 microns thick.
Description
BACKGROUND OF THE INVENTION
Cellulose acetate reverse osmosis membranes are produced by the
practice of such inventions as are described in U. S. Pat. No.
3,133,132 -- Loeb et al.; U. S. Pat. No. 3,432,585 -- Watson et al.
and U. S. Pat. No. 3,344,214 -- Manjikian et al.
Although it is reported in the literature that the structure of the
cellulose acetate appears to be a composite of a very thin dense
membrane and a thicker integral porous support substructure, it is
recognized by those skilled in the art that the porous region
actually displays graded porosity (the effective diameter of the
pores decreases in traversing the membrane in the direction of the
dense skin). The article "Preparation of Ultrathin Reverse Osmosis
Membranes and the Attainment of Theoretical Salt Rejection" by
Riley et al (Journal of Applied Polymer Science, Vol. 11, pages
2143-2158, 1967) reports an electron microscope study of the
structure of such membranes in which it was found that the dense
layer of cellulose acetate is about 0.2 microns in thickness and
the porous layer is formed integral therewith.
An improvement in the aforementioned method is described in the
article "Drying Cellulose Acetate Reverse Osmosis Membranes" by Vos
et al. (Industrial Engineering Chemistry -- Prod. Res. Develop.,
8(1), pp. 84-89, 1969) to overcome a problem also referred to in
the aforementioned patents. As stated in Vos et al, if such
membranes are allowed to dry without taking special precautions,
they suffer a non-recoverable loss in desalination and physical
properties. The Vos et al. method for overcoming this problem
prescribes soaking the membrane in a surface active agent.
Thereafter, the membrane may be permitted to dry out without
deleterious effect.
With respect to dry asymmetrical porous polymer structures, without
any dense surface layer, cellulose acetate membranes may be
prepared without annealing and drying using the Vos et al.
technique. This is the only known polymer material of which such
dry membranes have been prepared. Metal porous layers, that
approximate an asymmetric porous structure are disclosed in U. S.
Pat. No. 3,303,055 -- Gumucio wherein a multilayer metal electrode
construction is disclosed in which each succeeding layer presents a
change in porosity. In preparing the multilayer structure, layers
of particulate material are built up in the mold using different
particle size material for each of the various layers. The layers
of particulate material are then sintered to prepare an integral
unit. This porous structure is made of metal, rather than polymer,
and must have pores of relatively large dimension since this would
be a characteristic inherent from the method of manufacture.
A number of methods have been disclosed for the preparation of
porous polymers, however, none of these methods produce an
asymmetric structure. U. S. Pat. No. 3,378,507 -- Sargent et al.
reviews much of the patent art relating to the preparation of
porous polymer structures and is incorporated herein by
reference.
The art is in need of a method for the preparation of asymmetric
membranes of various crystalline polymer materials both with and
without the dense skin characteristic of the cellulose acetate
membranes. Further, the gas separation art is particularly in need
of integral structures of this type able to present very thin
non-porous layers of various polymers for perm-selective gas
separation of high efficiency.
SUMMARY OF THE INVENTION
An asymmetric microporous polymer membrane structure is produced by
this invention having a graded porosity, i.e. graded pore size
progressing from one major surface of the membrane to the other
major surface thereof. The structure will (depending upon the
conduct of the process) present porosity thereof as graded
interconnected pores either (a) ranging from an effective diameter
of about 100 A at one membrane face to a smaller undetermined size
adjacent a dense layer forming the second face or (b) ranging from
an effective diameter of about 100 A at one membrane face to some
very small finite value at the opposite face of the membrane
through which gas can freely pass.
The method of this invention for the preparation of such membranes
involves purely physical phenomena beginning with the selection of
a polymer, which can be cast in a crystalline form and is soluble
to different degrees in two mutually miscible solvents. The better
of the two solvents for the polymer must be more volatile than the
poorer of the two solvents. Next, a leaching agent is selected
having the properties of being miscible with both the mutually
miscible solvents and being a non-solvent for the polymer.
The method steps involve preparing a solution of the polymer in the
better solvent and then adding to this solution a quantity ranging
from about 10 to about 50 percent by volume (of the first solution)
of the poorer solvent to prepare the casting solution. A membrane
is cast from this polymer solution and is allowed to desolvate for
a short time before immersion thereof into the leaching agent.
Immersion time should be long enough for the membrane structure to
develop; that is, of the order of 1 minute or longer. After the
immersion the membrane is dried.
With many polymers that exhibit crystallinity it is possible,
depending upon the desolvation time employed, to produce either an
asymmetric microporous membrane having one thin dense major surface
(or skin) or an asymmetric microporous membrane without such a
dense skin.
BRIEF DESCRIPTION OF THE DRAWING
The practice of this invention and the products resulting therefrom
will be apparent from the following detailed description and the
annexed drawing displaying the graded porosity of the microporous
polymer membrane of this invention in cross-section.
DESCRIPTION OF THE PREFERRED EMBODIMENT
It has been unexpectedly found that by characterizing essential
features of the prior art methods employed in the preparation of
cellulose acetate reverse osmosis membranes in terms of the solvent
system employed and utilizing this characterization in connection
with polymers other than cellulose acetate, which exhibit
crystallinity, that not only is it possible to prepare asymmetric
microporous polymer membranes of these materials having a dense
thin surface layer but, as well, microporous graded structures
without the dense membrane may also be prepared. Surprisingly each
such structure may be dried without the necessity of employing a
surface active agent to reduce the surface tension of the leaching
agent or of employing liquid extraction or freeze-drying as are
described in the aforementioned Vos et al article.
Among the thermoplastic polymer materials from which asymmetric
microporous membranes may be prepared are the arylene oxide
polymers described in U. S. Pat. No. 3,350,844 -- Robb
(incorporated by reference), polycarbonate resins such as are
described in U. S. Pat. No. 3,256,675 -- Robb et al. (incorporated
by reference), polyvinyl acetate resins, polyalkyl methacrylate
resins, polysulfones, polymers of monovinyl aromatic hydrocarbons,
etc.
Both types of asymmetric microporous polymer membranes produced by
this invention have a number of applications. Thus, the asymmetric
membranes having a thin dense major surface are particularly useful
in gas separation because the composite membrane presents both (a)
an extremely thin non-porous film of the polymer membrane through
which gas permeation can be conducted and (b) backing support for
this thin membrane integral therewith and eliminating any necessity
to handle the very thin polymer film for placement on a separate
porous support. Asymmetric microporous membranes prepared without
the thin dense surface layer may be used to separate gaseous
components from a liquid e.g. in an artificial lung, as battery
separators, as support for thin-dip cast polymer membranes, as a
filter material, as an immobilizing structure for liquid membranes
employed in gas separation or as a component of a composite
structure useable to replace leather or fabric.
Once the crystalline polymer from which the membrane is to be
prepared has been selected, the solvent system to be employed may
be determined in routine fashion. Thus, a volatile (boiling point
of about 50.degree. C) solvent able to dissolve at least 15 percent
by weight of the polymer is first selected, if available. If such a
solvent is not available, a solvent having characteristics of
solubility and volatility as close thereto as possible is selected.
Next, (as related to this "good" volatile solvent) a poorer solvent
for the same polymer, which is (a) less volatile than the good
solvent by a factor of at least about 2, and (b) is miscible with
the good volatile solvent is selected in order that a casting
solution may be prepared containing these three components; namely,
the polymer material, the good volatile solvent, and the poor
non-volatile solvent.
The characterization of "good" and "poorer" with respect to the
aforementioned solvents illustrates a difference in the capability
to dissolve polymer ranging from about 2 to about 10 times, e.g.
the "good" solvent should be able to dissolve from about 2 to about
10 times the amount of polymer as will dissolve in the "poorer"
solvent.
A similar routine method of selection is employed in determining
the liquid leaching agent. This material must (a) be miscible with
both the good solvent and the poor solvent, but (b) be a
non-solvent for the polymer material.
Having selected the polymer, the solvent system and the leaching
agent, the procedure for the preparation of the asymmetric
microporous membrane is as follows: at least about 10 percent by
weight of polymer is dissolved in the good, volatile solvent. A
solution is then prepared consisting of about 1 to 9 parts of the
polymer solution to 1 part of the poorer, less-volatile solvent. A
membrane of this solution is cast upon a flat surface (e.g. a glass
plate) in the conventional manner employing a doctor blade with the
blade setting ranging from about 5 to 25 mils from the flat
surface. The procedure is very straightforward and casting may be
accomplished in air at ambient temperatures ranging from about
20.degree. to 30.degree. C. Special atmospheres are not
required.
The decision must be made, of course, as to whether or not the
asymmetric membrane is to have a dense major surface or not. Having
decided which of these structures is desired, the next three steps
constitute desolvation (which introduces a variable, desolvation
time, in accordance with this selection), immersion in the leaching
agent and then drying of the membrane. The desolvation time
necessary for production of the desired asymmetric microporous
structure is determined in a routine fashion by employing a range
of desolvation times for a series of samples and carrying out the
remaining process steps with each of the various samples.
Thus, a very short desolvation time, e.g. 5 seconds may suffice for
the preparation of an unskinned asymmetric microporous membrane
while desolvation times in excess of about several minutes may be
required to produce a skinned asymmetric microporous membrane of
the same polymer material. The resulting membrane is typically a
white, opaque membrane having an area reduced by about 10% from the
area of the membrane as cast and having a thickness of about 5 to
25 percent of the membrane as cast.
In the case of the skinned asymmetric microporous membrane, the
surface thereof disposed away from the casting surface during the
casting operation becomes the dense layer and is slightly more
shiny than the opposite major surface. As is shown schematically in
the drawing, the membrane consists of discrete crystals held
together by some interconnecting mechanism, which is as yet
unknown, representing a relatively stiff system defining a
structure having a graded porosity in which the effective diameter
of the pores increases with distance traversed from face A to face
B. The dense non-porous skin of face A may vary from about 100 A.
to about 10 microns in thickness.
Because of the crystalline nature of the microporous polymer
structure no difficulty has been experienced with problems of cold
flow and/or structural collapse as has been stated as being a
problem in the preparation of some homogeneous microporous
structures. With respect to drying of the membrane this significant
crystallinity makes unnecessary any modification of the leaching
agent as by the addition of surface active agents.
The change of porosity with distance through the membrane varies
for different polymers, different casting solutions, different
casting conditions and different desolvation conditions. Also, in
the case of skinned asymmetric microporous polymer membranes, the
initial thickness of the cast film has an effect on the final
product in that for a given desolvation time a thicker initial film
as cast will result in a somewhat thinner skin. The series of
variables recited provides a capability for simply and controllably
determining the end product. Once a given set of conditions is
fixed, the results are reproducible.
The variables effect the process and product as follows:
a. Desolvation Time -- shortening of the desolvation time reduces
the skin thickness and eventually leads to the production of an
unskinned microporous membrane while sufficient lengthening of this
variable will eventually produce a non-porous skin with many
polymers useful in the practice of this invention;
b. Temperature -- conduct of the process at lower temperature acts
to increase the desolvation time and vice versa;
c. Solvent System -- different solvent systems will contribute
different desolvation times;
d. Initial Thickness of Cast Film -- for a given desolvation time a
thicker film as cast results in a thinner (or non-existent)
skin.
Suggested solvents and leaching agents for particular polymer
materials are, by way of example:
Good Poorer Leaching Polymer Solvent Solvent Agent polymethyl
acetone formamide water methacrylate polystyrene acetone formamide
water copolymer of chloroform toluene methanol bisphenol-A and
dichlorodiphenyl- sulfone
In some instances it is preferable to employ co-solvents as the
"good" solvent e.g. when it is necessary to dissolve a polymer,
which has both polar and non-polar constituents.
This invention offers particular promise in the preparation of gas
separation membranes enabling the production of composite
integrated structures having a very thin effective thickness for
the permeation medium using less stringent conditions of
cleanliness than are necessary, for example, in the solvent casting
of ultrathin films. This type of casting of films that are
subsequently supported on porous substrates is described and
claimed in U. S. Pat. application Ser. No. 763,879 -- Ward et al.
(Composite Permselective Membrane Structure and Preparation
Thereof) filed Sept. 30, 1968, now abandoned and assigned to the
assignee of the instant invention.
EXAMPLE 1
A 10 percent solution of polyxylylene oxide in chloroform was
prepared, chloroform being the good, volatile solvent. Next, a
solution was prepared consisting of 2 parts of the polyxylylene
oxide solution to 1 part dichlorobenzene, the poor, non-volatile
solvent.
A membrane was cast in air at ambient temperature and pressure on a
glass plate using a doctor blade with a blade setting of 10 mils.
The cast membrane was allowed to desolvate for 30 seconds and was
then immersed in methanol, the leaching agent.
The membrane was dried.
The resultant product was an opaque microporous membrane of about
1.3 mils in thickness. On visual inspection it appears that the
side away from the glass plate during casting was slightly shinier
than the side toward the glass plate.
On subsequent testing for permeating properties the O.sub.2 and
N.sub.2 permeabilities for the dried membrane were,
respectively,
18.5 .times. 10.sup..sup.-9 and 4.22 .times. 10.sup.-.sup.9 (cc gas
RTP, cm thick/(sec, sq cm, cm Hg .DELTA.P)).
The permeability values for normal non-porous polyxylylene oxide
membranes for O.sub.2 and N.sub.2 are, respectively,
1.7 .times. 10.sup..sup.-9 and 0.35 .times. 10.sup..sup.-9 (cc gas
RTP, cm thick/(sec, sq cm, cm Hg .DELTA.P))
respectively. A comparison of the above permeability values
establishes that the effective thickness of the whole asymmetric
polyxylylene oxide membrane was about 2.7 microns, which indicates
a skin thickness of 2.7 microns or less. The mathematics of this
comparison is as follows: ##SPC1##
The oxygen-to-nitrogen permeability ratios for both the membrane
prepared in Example 1 and normal non-porous polyxylylene oxide
membranes is substantially constant thereby verifying the presence
of a non-porous skin.
EXAMPLE 2
Polycarbonate resin was dissolved in methylenechloride to form a 20
wt % solution. The solution is then mixed with toluene in a volume
ratio of 2 to 1. The methylenechloride is the good, volatile
solvent and the toluene is the poor, non-volatile solvent.
A membrane was doctor blade cast in air (ambient conditions) on a
glass plate at a liquid thickness of 20 mils and was allowed to
desolvate in air for one minute. The glass plate and membrane were
then immersed in methanol and allowed to "gel".
The resultant product was a white, opaque membrane 4 mils thick
with a nitrogen permeability of
0.62 .times. 10.sup..sup.-9 (cc gas RTP, cm thick)/(sec, sq cm, cm
Hg .DELTA.P) and
Pr.sub.O /Pr.sub.N = 4.5. The accepted "dense membrane" values for
polycarbonate are Pr.sub.N = 0.037 .times. 10.sup..sup.-9 and
Pr.sub.O /Pr.sub.N = 4.6. Thus the effective thickness of the 4-mil
asymmetric film is calculated to be 0.24 mil. As is indicated in
Example 1, this is an indication of the skin thickness. The balance
of the membrane was made up of microporous (graded porosity)
crystalline structure.
EXAMPLE 3
The same casting solution as in Example 2 was cast but the
desolvation time was reduced to 30 seconds. The resultant white,
opaque, 4-mil film had a nitrogen permeability of 14.7 .times.
10.sup..sup.-9 and Pr.sub.O /Pr.sub.N = 0.92. The low separation
factor indicates that no skin developed, but the asymmetric
microporous structure was present.
EXAMPLE 4
A polyvinyl butyrate solution of the following is prepared:
7.5 gm polyvinyl butyrate
8 cc glacial acetic acid
34 cc methyl ethyl ketone (MEK)
7 cc formamide
Acetic acid and MEK constitute good co-solvents and formamide is
the pore-forming poor solvent. It has been found that casting
solutions containing formamide are unstable. Unexpectedly, acetic
acid (in addition to its solvent action) acts as a stabilizer for
the solution.
Membranes were cast in thicknesses of three mils, desolvated, and
gelled in water. The results of subjecting these identical films to
different desolvation times are shown in the following table.
##SPC2##
It is apparent that as desolvation time increases there is an
increased degree of skin formation as seen by changes in Pr.sub.O
/Pr.sub.N . However, the desolvation time was not long enough to
develop a dense skin on the membrane using this particular solvent
system. By way of comparison, a non-porous film of polyvinyl
butyrate exhibits values as follows:
0.077 .times. 10.sup..sup.-9 (cc/cm.sup.2 sec) (cm thick/cm Hg
.DELTA.P) for the
nitrogen permeability and 2.6 for the Pr.sub.O /Pr.sub.N .
* * * * *